Field of the Invention
The present invention relates to an external resonator type laser device.
Description of the Related Art
Conventionally, a wavelength conversion type laser in which a surface emitting type semiconductor device is provided with an external resonator, and a wavelength conversion element formed of nonlinear optical crystals is interposed between the semiconductor device and the external resonator is known. A technique of using a wavelength selectable reflecting mirror as the external resonator, and optical crystals of polarity-inverted lithium niobate (PPLN: Periodically Poled Lithium Niobate) or the like as the wavelength conversion element is known (see Patent document 1 below). Also a technique of using a volume bragg grating (VBG) in place of the reflecting mirror used in an external resonator is known (see Patent document 2 below). Also a technique of using a TFF (dielectric thin film filter) and an optical member in place of the volume bragg grating (VBG) is known (see Patent document 3 below).
Referring to FIG. 9, a conventional external resonator type laser device (hereinafter, appropriately abbreviated as a “laser device 90”) will be described. FIG. 9 is a view schematically showing a configuration of a conventional external resonator type laser device disclosed in Patent document 2.
The conventional laser device 90 includes a semiconductor device 3 having a light emitting part 2, a volume bragg grating (VBG) 5 as a wavelength selection element, periodically poled lithium niobate (PPLN) 7 as a wavelength conversion element, and reflection members 9, 11.
As shown in FIG. 9, the light emitting part 2 is disposed on the semiconductor device 3. Basic light is radiated from the light emitting part 2. The semiconductor device 3 is brought into abutment on a heat sink 15 for radiating the heat generated from the light emitting part 2.
The basic light radiated from the light emitting part 2 is light having a predetermined wavelength distribution. The VBG 5 is provided for selecting a specific wavelength range from the basic light. The PPLN 7 has a function of outputting light after conducting wavelength conversion on part of the light in a specific wavelength range, of the incident light.
The light emitting part 2 installed on the semiconductor device 3 radiates basic light. The basic light is not light that has only components of a specific wavelength range, but light having a predetermined wavelength distribution. As the basic light passes the VBG 5, only the light in a specific wavelength range is selectively reflected. Hereinafter, the light selected by the VBG 5 as the wavelength selection element is called “first light”. The VBG 5 has a function of transmitting light in a wavelength range of later-described second light.
The first light reflected from the VBG 5 passes inside the PPLN 7. The PPLN 7 is configured to output light after conducting wavelength conversion on part of the light in a specific wavelength range as described above. Here, the PPLN 7 converts part of wavelengths of the light of the wavelength components possessed by the first light, and generates light having a different wavelength. Hereinafter, the light converted by the PPLN 7, and having a wavelength after conversion is called “second light”.
Even when the wavelength range of the first light is in a wavelength range that can be wavelength-converted by the PPLN 7, the entire incident first light is not converted into the second light at once, but part of the first light passes the PPLN 7 without being converted. Therefore, from the PPLN 7, the second light generated by the PPLN 7, and the first light that is not wavelength-converted in the PPLN 7 are outputted toward the semiconductor device 3.
Between the PPLN 7 and the semiconductor device 3, the reflection members 9 and 11 are disposed. These reflection members 9 and 11 are configured to reflect the light of the wavelength range possessed by the second light, and transmit at least the light of the wavelength range possessed by the first light. Therefore, the second light outputted from the PPLN 7 toward the semiconductor device 3 is reflected by the reflection members 9 and 11 and the direction of the light is changed, and then outputted outside the device 90 (second light 40).
On the other hand, the first light that is not wavelength-converted by the PPLN 7 and has permeated the PPLN 7 as it is permeates the reflection member 9 and enters the semiconductor device 3. The semiconductor device 3 is provided with a reflection member (internal mirror) (not shown), and the incident first light is reflected toward the direction of the PPLN 7.
The first light that is incident from the side of the semiconductor device 3 is partly wavelength-converted while it passes inside the PPLN 7 to become second light in the same manner as described above. The second light reaches the VBG 5. As described above, since the VBG 5 is configured to transmit the light of the wavelength range possessed by the second light, the incident second light permeates as it is and is then outputted outside the laser device 90 (second light 50). On the other hand, the first light that is not wavelength-converted is reflected again by the VBG 5 and travels toward the PPLN 7.
That is, in the laser device 90, an external resonator is formed between the internal mirror formed inside the semiconductor device 3 and the VBG 5, and the first light is repeatedly reflected between these. While the reflection is repeated, the first light having passed the PPLN 7 is sequentially wavelength-converted into the second light, and taken out outside the device 90 from the VBG 5 or the reflection member 11.
That is, the laser device 90 is configured to select the first light having a first wavelength range selected by the VBG 5 from the basic light radiated from the light emitting part 2, and generate the second light that is wavelength-converted by the PPLN 7 and has a second wavelength range, from the first light, and output the second light outside as intended laser light. As one example, the first light can be infrared light having a peak wavelength of about 1065 nm, and the second light can be green visible light having a peak wavelength of about 532.5 nm.